This invention relates to the measurement and identification of species in a flowing gas sample, and, more particularly, to a miniature device for performing such determinations.
A person may be exposed to small amounts of species, such as organic agents, that may be quite harmful to the person""s health. For example, a soldier may be exposed to chemical warfare agents, a chemical worker may be exposed to leakages or spills, or a rescue worker may enter a situation where unknown chemical agents, some of which may be harmful, may be present. In each such situation, it is important to know whether any such species are present and, if so, the nature of the species so that the person may take the necessary precautions.
Where the concentrations of the species are relatively large, they may often be detected by the senses of the person, such as a characteristic smell, or a physiological reaction of the person, such as watering eyes. However, many species either cause no such reactions, or cause harm even when present in concentrations below the human detection level. A measurement device is required for determining the presence of these very low concentrations, and such a device may be necessary even if the person detects or has a reaction to the species, in order to identify the exact species.
In a known type of ion mobility spectrometer used to determine the presence of species, a sample of air or other gas, usually heated, is ionized and passed through a flow channel. A transverse electric field deflects the ionized species. The amount of deflection depends upon several factors, including, for example, the size of the ion and its electrical charge, the air flow rate, and, secondarily, the mass of the molecule. Electrodes positioned in the wall of the flow channel collect the ions, and the ion current flow at each electrode is a measure of the number of ions reaching that electrode. From the distribution of the electrode currents and pre-established calibration information, information regarding the nature of the species may be deduced.
Existing M-90 ion mobility spectrometers, which may operate from a battery, are rather large in size, about 11 inches by 11 inches by 4 inches, and heavy, about 15 pounds (including battery pack), and are thus too large to be considered a xe2x80x9cpersonalxe2x80x9d instrument to be carried in the pocket. The M-90 instrument has only two sets (channels) of three electrodes each, limiting the possible chemical resolution of the instrument. Large air flow rates (up to 2 liters per minute) are used, requiring large power consumption to heat the air and force it through the flow channel. The M-90 ion mobility spectrometer has a relatively high power consumption, requiring a large, bulky battery pack when used in the self-contained mode.
To be considered a xe2x80x9cpersonalxe2x80x9d-sized ion mobility spectrometer for at least some applications and as used herein, an ion mobility spectrometer must have a total volume of less than about 40 cubic inches, a weight of less than about 2 pounds (including battery pack), and an operating life of at least about 12 hours from a portable battery power source without recharging or replacing the battery. The existing devices cannot meet this requirement. Other types of gas flow analysis devices that meet these requirements do not have the operating advantages of the ion mobility spectrometer.
Thus, there is a need for a miniature, personal-sized ion mobility spectrometer. The present invention fulfills this need, and further provides related advantages.
The present invention provides the first miniature, personal-sized ion mobility spectrometer for the identification of chemical species in a gas sample. A preferred form of the ion mobility spectrometer of the invention has approximate dimensions of 6 inches by 3 inches by 2 inches and weighs less than 2 pounds.
The ion mobility spectrometer of the invention is more sensitive and has higher selectivity than prior devices operating according to the principle of ion mobility spectrometry. Due to the higher sensitivity of its physical structure and the electronics, the ion mobility spectrometer of the invention may operate using lower air flow rates and has a longer operating life on a smaller single set of batteries prior to recharging or replacement. The ion mobility spectrometer may therefore be readily carried in a hand or a pocket of the user. It may be powered by a conventional battery pack commonly used for personal-sized, hand-held instruments, such as a BA5800/U battery pack issued by the US Government. Additionally, the analysis of chemical species is more precise than possible with existing ion mobility spectrometer devices, because larger numbers of sensing elements and two-dimensional arrays of sensing elements may be incorporated.
In accordance with the invention, an ion mobility spectrometer comprises a spectrometer cell having a gas inlet, a gas outlet, and a flow channel therebetween. The flow channel has a wall, a gas inlet end at the gas inlet, and a gas outlet end at the gas outlet. A downstream direction, the direction of air flow, is defined from the gas inlet to the gas outlet, and an opposite upstream direction is defined from the gas outlet to the gas inlet.
A field source of an electric field, including opposed, facing pairs of electrodes positioned at the facing walls of the flow channel, establishes an electric field over a measurement region of the flow channel. The electric field is perpendicular to the local direction of air flow. The sensor electrodes on one side of the flow channel, which form a part of the sensor elements, are preferably at or near ground potential. The opposing field electrodes that are paired with these sensor electrodes are connected to a power supply to provide them with an electrical potential. In this way, electric fields are established between pairs of electrodes in the measurement region. Equivalently, the multiple field electrodes may be provided as a single field electrode extending over the entire measurement region.
For most applications, at any moment in time all of the electric fields are preferably of the same polarity and oriented in the same direction, providing a nearly uniform field through which the ions migrate. However, other approaches may be desirable in other circumstances. For example, the electric fields may be of mixed polarity, such as with the leading electrodes first encountered by the gas flow of one polarity and the remaining electrodes of opposite polarity. In another example, it may be desirable to have a nonuniform field strength, such as with a lower voltage field produced by the leading electrodes and a higher voltage field produced by the remaining electrodes.
A plurality of sensor elements utilize the plurality of respective sensor electrodes (those which are at or near ground potential), and an electrically communicating readout circuit array integral with or attached to the plurality of sensor electrodes. The readout circuit detects and amplifies the ion current reaching each of the sensor electrodes. The readout circuit array comprises an integrated circuit operable to detect an electrical charge accumulation or ion current on each of the sensor electrodes. An important feature of this invention is the use of monolithic pre-amplification circuitry. This readout device contains one preamplification circuit for each sensor electrode or channel. The preamplification circuits are optimized for low noise and amplification of low currents.
The ion mobility spectrometer further includes an air pump operable to force gas through the flow channel, a gas heater in the flow channel upstream of the source of the electric field and the plurality of sensor elements in the measurement region, and an ionization source in the flow channel upstream of the source of the electric field and the plurality of sensor elements.
The personal-sized ion mobility spectrometer has a size of less than about 40 cubic inches and a weight of less than about 2 pounds (including the battery), and is operable on battery power for at least 12 hours before replacement or recharging of the battery. The readout circuit electronics of the ion mobility spectrometer is sensitive to ion currents of less than about 1 picoampere.
In operation of the ion mobility spectrometer, a gas such as air is drawn into the gas flow channel and heated to a preselected constant temperature. The heated gas is passed through an ionizing region, so that any organic or other species to be detected are ionized. The gas, possibly containing such ionized species, is passed through the electric field, which causes the ionized species to be angularly deflected according to the sense and magnitude of the electric field. The amount of deflection also depends upon the size, charge, mass, and other properties of the species, as well as the flow rate of the gas. In general, though, larger species are deflected less than smaller species, so that the smaller species contact the sensor electrodes on the wall of the gas flow channel upstream of the larger species. As the ions strike the sensor electrodes in the wall, their charge is transferred to the sensor electrodes and an electrical current flows in the preamplification readout circuit. The ratios of the current flows at the various sensor electrodes forms a pattern that reveals the type of ionized species in the air. The magnitude of these current flows is related to the concentration of these species in the flowing air. From this information and calibration data obtained using known amounts of known species, the single or multiple species in the flowing sample may be determined. See, for example, U.S. Pat. No. 5,047,723, whose disclosure is incorporated by reference. The calibration information is preferably stored in look-up tables in a memory of the ion mobility spectrometer, so that identification of unknown species may be made as the data is gathered.
The polarity of the electrodes may be fixed. In another embodiment, the polarity of the electrodes may be periodically changed so as to force ions of positive charge to the sensor electrodes during a first period of time, and later to force ions of negative charge to the same sensor electrodes during a second period of time. In this case, the positively and negatively charged ions would be detected alternately by the one set of sensor electrodes.
The sensor electrode array may be separate from the readout chip, or may be an integral part of it. The use of an integrated electrode and readout circuit allows the sensor electrodes to be made much smaller than previously possible in devices of this type. The use of smaller sensor electrodes leads to much higher spatial resolution of the mass distribution. Different types of sensor electrode arrays may be used, so that multiple types of data may be gathered. Additionally, the smaller sensor electrodes allow the gas flow rate to be reduced, reducing the amount of air that must be drawn through the gas flow channel and heated, and therefore the power requirements of the device. This, in turn, allows the gas flow channel to be smaller in cross section than previously possible. The use of the smaller sensor electrodes also allows the use of a two-dimensional array of sensor electrodes within the gas flow channel without enlarging the size of the cell. A two-dimensional array of sensor electrodes may be used to gather more information about the ionized species in the gas flow than can a one-dimensional array.